1. Field of the Invention
The invention relates to a method for multicolor printing of stock or printing materials which are in web or sheet or plate form and have a smooth, nonabsorbent surface, such as plastic or metal plates, foils, or plastic-coated papers, and so forth, and a printing press for printing in accordance with the foregoing method.
In the conventional wet offset method, such materials can be printed only with limitations. It would indeed be possible to print these materials by the waterless offset process. However, the silicone surface of the dry offset plates (Toray plates) is only conditionally resistant to the required UV inks. The service life of the plates is shortened by ultraviolet or UV inks. The vulnerability or sensitivity of the silicone layer of waterless offset plates is contradictory to the required UV inks and to the drying process or the process of anchoring the inks to the surface of the plastic material.
In multicolor wet on wet printing, on the other hand, it is not possible to achieve especially high quality with UV inks on plastic material.
The aforementioned materials are therefore generally printed at the present time by flexographic printing, sometimes also with inks that are cured with the aid of UV projectors immediately after the printing process. For example, the book entitled "Technik des Flexodrucks" [Flexographic Printing Technology] published in 1982 by DFTA-Flexodruck Fachgruppe e.V. discloses the printing of such cylindrical containers as cans, tubes, and so forth, in multiple colors with UV-curable inks by flexographic printing. The machine or press described in this book on page 12.7 thereof has a plurality of printing units having UV projectors disposed therebetween, and are said also to be capable of printing by the indirect letterpress method, i.e., the letterset process. Such machines are very expensive, however, because they are built only by special order and are not very worthy except for very large printing jobs and when they are in use very frequently.
Moreover, limits are also set to the print quality, due to the use of letterpress methods. One problem, for example, is that printing ink builds up on the flexographic printing form and collects in the indentations on the flexographic printing form, respectively, so that there are isolated instances of ink being transferred to nonprinting regions of the flexographic printing form.
German Patent 15 71 863 discloses a simple, direct letterpress method wherein the residual film of ink remaining after the printing operation is removed completely each time before the printing form is re-inked. The residual ink film is removed from the printing form via ink removal rollers and returned to the ink supply again. However, this German patent makes no reference to the problems which arise in multicolor printing especially on nonabsorbent surfaces.
Canadian Patent 1,251,688 describes a lithographic offset printing press with a short inking unit and a dampening unit, wherein excess ink is scraped from the wet offset plate with a doctor blade.
2. Summary of the Invention
It is accordingly an object of the invention to provide a method for multicolor printing of nonabsorbent material by which stock or printing materials in web or plate form with a nonabsorbent surface can be printed in multiple colors simply and economically with high quality, and to provide a printing press suitable for performing this method.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a method for multicolor printing of stock in web or plate form and having a smooth, nonabsorbent surface, which comprises printing on the stock by the indirect flexographic printing process, assigning each color to be printed to a respective printing unit which includes an inking unit, a cylinder for a flexographic printing form, a transfer cylinder and a counterpressure cylinder, always drying the ink on the stock before passing the stock to the next printing unit, and removing ink located in indentations and on sides, respectively, of the printing surfaces of the flexographic printing form.
In accordance with another mode, the method of the invention includes printing with UV-curable inks and, downstream of each printing unit, irradiating the printed stock with UV light.
In accordance with a further mode of the method of the invention, the printing form is a metal-lined flexographic printing plate.
In accordance with an added mode, the method of the invention includes applying the ink by a conventional offset inking unit.
In accordance with an additional mode, the method includes applying the ink with a short inking unit to the flexographic printing plate.
In accordance with yet another mode, the method of the invention includes applying the ink to the flexographic printing plate with a screen roller cooperating with a chamber doctor blade.
In accordance with yet a further mode, the method of the invention includes removing the ink in the indentations and on the sides, respectively, of the printing surfaces of the flexographic printing form and reapplying that ink as a thin film before each re-inking of the printing form.
In accordance with yet an additional mode, the method of the invention includes completely cleaning the flexographic printing form a multiplicity of times during an ongoing printing job.
In accordance with yet an added mode, the method of the invention includes maintaining the flexographic printing form in continuous contact with a soft roller during the printing process.
In accordance with another aspect of the invention, there is provided a printing press for multicolor printing of stock in web or plate form and having a smooth, nonabsorbent surface, comprising a plurality of individual printing units, respectively, associated with a single color, the printing units being arranged in rows and having at least one inking unit, one cylinder for a flexographic printing form, one transfer cylinder and one counterpressure cylinder, a plurality of drying devices for the printing ink, respectively, assigned to one printing unit, and a plurality of devices assigned to the flexographic printing forms for removing ink from the indentations and sides, respectively, of the printing surfaces of the applicable flexographic printing form, the ink-removing devices being actuatable for removing the ink continuously or repeatedly at short intervals during a printing job.
In accordance with another feature of the invention, the printing press includes a control system connected to the device for removing ink and having an input for introducing the cleaning intervals.
In accordance with a further feature of the invention, the drying devices include UV projectors.
In accordance with an additional feature of the invention, the printing press includes a wet offset printing press rebuilt for the letterset process, wherein a groove in the printing form cylinder for receiving the flexographic printing form is enlarged to between 1 and 2 mm, and into which, instead of the dampening units, one device for removing ink from the indentations and sides, respectively, of the flexographic printing form is built into each of the individual printing units.
In accordance with yet another feature of the invention, the device for removing the ink is a washing device.
In accordance with yet a further feature of the invention, the device for removing the ink is a roller with a soft, ink accepting surface installed instead of a dampening fluid applicator roller.
In accordance with a concomitant feature of the invention, the printing cylinders in each printing unit and the transfer cylinders between the printing units have at least twice the diameter of the printing form and transfer cylinders, respectively.
In this manner, it is possible to print on platelike plastic material, of the type used for check cards, telephone cards, chip cards, and so forth, quickly and inexpensively with a quality previously thought unattainable, because the printing press required for performing the method can be assembled by modifying printing units built in great numbers for wet offset printing. All that needs to be done is to change the depth of the recess in the plate cylinder from a typical value of 0.3 to 0.5 mm to a value of 1 to 2 mm, so that instead of the wet offset plates the flexographic printing plates, which are thicker, can be fastened thereon. Instead of the dampening unit which is otherwise customarily present, a device for removing ink from the indentations or sides of the printing surfaces of the flexographic printing plate is then installed: either a washing device, or a roller with a soft, ink-accepting surface provided instead of a dampening fluid applicator roller which is otherwise present. Finally, if UV-curable inks are used, suitable drying units, i.e., UV projectors suitably dimensioned with respect to power, can be installed in the printing unit.
By dispensing with dampening, stability in ongoing production printing is rapidly attained, and spoilage upon startup is drastically reduced which, for the relatively costly plastic materials makes for a substantial cost savings and improvement in quality.
The inking unit, conversely, can either be kept with a short inking unit or replaced with a screen roller which cooperates with a chamber doctor blade.
Depending upon the particular printing requirement, it may be expedient for the flexographic printing form to be cleaned repeatedly at relatively shorter or longer intervals during an ongoing printing job, or even before each re-inking operation, so as to assure always replicable conditions for every impression. It is therefore advantageous to provide a suitable control system for that purpose, into which the cleaning intervals can be input. In this way, the time needed to perform a printing job can be suitably optimized for the printing quality desired.
Especially whenever the ink residues are to be removed before each re-inking of the printing form, it is expedient to accomplish this with a soft roller which rests in continuous contact on the flexographic printing form and removes excess ink residues from the printing form, and then, after subsequent distribution by a second roller, re-applies the ink in a thin, even film.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for multicolor printing of nonabsorbent material, and a printing press for printing in accordance with the method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.